1. Field of the Invention
The present invention relates to bridges, and more particularly, to a cable stayed suspension bridge
2. Description of the Prior Art
Conventional suspension bridges are usually of the catenary type. Viewed from the side, the catenary bridge includes a cable suspended, in a catenary curve, between two towers, and a series of vertical cables suspending the deck of the bridge to the catenary cable The outline defined by a pair of adjacent vertical cables, with the respective portion of the catenary cable and the deck, is that of a quadrilateral polygon A quadrilateral is an unstable shape for a frame, leaving catenary bridges well known for instability, subject to deformation waves that reflect from end to end of a bridge. When periodic loads, such as wind gusts, correspond to a natural harmonic of a catenary bridge, the resultant resonance can pose a danger to the bridge. Therefore, heavy trusses are added for stiffness, but their added weight and cost do not contribute directly to bridging a gap. The longer the span, the greater the possibility of harmful vibrations, and the quadrilateral, therefore, imposes limits on possible catenary spans.
A less known type of suspension bridge is a stayed bridge. West German Auslegeschrift 1,235,973, published Mar. 9, 1967, and U.K. Patent Application GB 2,109,040 A, published May 25, 1983, describe one such type of stayed bridge, while Swedish Patent 179,453, published Mar. 29, 1962, illustrates a more complex stayed suspension bridge. Both of these types of stayed bridges have, in side view, a series of vertical outlines shaped as triangles, each framed by a stay, a tower, and a deck portion. A triangle is a stable shape for a frame, assuming stiff sides or at least no compression in a flexible side, leaving stay bridges well known for stability. However, a disadvantage of existing stayed bridges is longitudinal compression in a deck that requires compressive capacity to be added to a deck, but the added weight and cost do not contribute directly to bridging a gap. The longer the span, the greater the deck compression, and the compression, therefore, imposes limits on span lengths with stays.
A bridge, whether catenary or stayed or any other type, has an obvious need to support its own dead weight plus live loads including wind and earthquake loads. Also, any bridge has a limit of span imposed by a given design based on a limit of strength imposed by given materials, of construction.
The disadvantage of quadrilateral instability, and the resultant need for a weight of stiffening trusses, all restrict a conventional catenary span to 5 to 10 times the height of tower above deck.